Chris wrote:
You are going to get at least a little circulating ground current because of power supply parasitics
There is a reason why some power transformers cost $385 and others with similar basic specs (voltage, current) cost $22. Properly designed instruments, radios, medical and other equipment (even quality audio and video gear) that must interoperate in adverse circumstances use the expensive ones, for good reason.
I agree that for the typical test equipment case where all the gear is running from the same power feed it likely should not be necessary. But putting the connectors on opposite sides of the PCB is still just asking for trouble.
I think those are two different things (ground currents and RF currents). If the shields of all of the incoming and outgoing signal connectors are bonded firmly to a small area of the metal enclosure, and the enclosure is an effective shield at the RF frequencies of interest, there will be very little to no RF current on the shields to be drawn across the PCB. If the internal power supplies are single-point grounded there, as well, and the parasitics are kept low and balanced (you used the $385 transformer, and best design practices), no mains power-related current will be originated on the shields by the device.
Even a moderately complex instrument has signals coming and going not just to different areas of one PCB, but to several (or even dozens) of different PCBs that may use different power supplies. I'm not saying it's a bad idea to put each PCB's IOs in one small area of the card, but that is just one way to get where you need to be.
To maximize the probability that the conditions above are met (in particular, to maximize the effectiveness of the enclosure as a shield), you can add ferrite common-mode chokes to both the internal and external coax cables feeding the IO connectors. Use lots of fasteners to assemble the enclosure, make sure each one provides good metal-to-metal bonding (use masks when the various enclosure parts are painted), and use RFI gaskets, spring fingers, wire mesh shaft seals, etc. as needed. Look critically at every hole you are forced to make in the enclosure, and use whatever means are necessary to make them RF-tight at all frequencies at which your circuitry might be vulnerable.
Best regards, Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
